1. Field of the Invention
The invention relates to a plant for producing cyanuric acid by pyrolysis of urea, comprising a first rotary cylindrical reactor in which the urea cyanurate balls are formed and a second reactor where said pyrolysis takes place.
The invention also relates to a process for the preparation of cyanuric acid, comprising a first step of preparation of urea cyanurate by reaction, in a first rotary reactor, of urea with recirculated cyanuric acid and a second step of pyrolysing the urea cyanurate in a second reactor.
2. Description of the Prior Art
Cyanuric acid is prepared on an industrial scale from urea by pyrolysis thereof, with or without a solvent, as per the following reaction: EQU 3 H.sub.2 NCONH.sub.2 .fwdarw.C.sub.3 H.sub.3 N.sub.3 O.sub.3 +3 NH.sub.3
The processes using a solvent have the common drawback of the recovery thereof and also the subsequent environmental problem. This means that they are not economically advantageous. The dry methods are preferable, although here the problems arise from the soiling of the reactors during pyrolysis. The soiling is of such a degree that, unless precautions are taken, it reaches the extreme of invalidating the process. Several processes have been devised to solve this problem. Some use an acid catalyst and the majority try to increase the heat exchange in some way, either by mixing the urea with a molten metal, by fluidisation or, more frequently, by using a tubular reactor.
These reactors must be provided with a blade system scraping the wall thereof to remove the furring which inevitably forms. Such reactors are described in U.S. Pat. No. 2,943,088, FR 1,183,672, ES 520,763 and U.S. Pat. No. 4,474,957. Nevertheless, these reactors, although to a lesser extent, continue to give soiling problems obliging the plants to be shut down periodically for cleaning. The process is improved if the urea cyanurate is prepared prior to pyrolysis by reaction of the cyanuric acid on the molten urea, which is then subjected to the pyrolysis as such. This process may be carried out in two different reactors (U.S. Pat. No. 3,318,887) or in a single one having different temperature zones (ES 540,265), or in such a way that the molten urea is distributed through different inlets in the furnace (U.S. Pat. No. 4,474,957). Of these options, the last two require much more complex control instrumentation and a larger furnace also, leading to high investment and maintenance costs. The first option is preferable, since although it requires two reactors, the first one is very simple to construct and the second one carries out the pyrolysis integrally under particular fixed conditions, whereby the control of the operative conditions is simplified.
U.S. Pat. No. 3,318,887 gives as an example of a reactor in which the urea cyanurate may be formed a rotary drum provided with fixed internal blades. The patent likewise teaches that the working temperature in the first reactor should range from 125.degree. to 160.degree. C. A drawback of this process is the formation of lumps, since the balls are scarcely well formed or, if they are, they break on colliding against the blades. A time also comes with this plant when it must be shut down for cleaning purposes.
Another drawback in the manufacture of cyanuric acid is to be found in the scrubbing column for the gases exhausting from the reactor. Apart from the ammonia formed in the reaction, these gases also entrain sublimated urea, finely divided cyanuric acid and a number of products derived from the reaction, such as cyanic acid, ammonium cyanate and carbamates. These products accompanying the NH.sub.3 finally block the scrubbing column, in spite of the improvement introduced in U.S. Pat. No. 2,943,088, consisting of scrubbing the gases with a current of hot urea, and this means that the plant must be shut down for cleaning.